1. Field of the Invention
The present invention relates to a method of making biasable system layers or films for use in an electrostatographic, including digital, apparatus. More in particular, the present invention relates to a method of making a polyimide layer containing fluorinated carbon materials, which layer finds particular utility as a substrate of an intermediate transfer or transfix member of an electrostatographic device.
2. Discussion of Background Art
Generally, the process of electrostatographic copying is initiated by exposing a light image of an original document onto a substantially uniformly charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges a photoconductive surface thereon in areas corresponding to non-image areas in the original document while maintaining the charge in image areas, thereby creating an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by depositing charged developing material such as toner onto the photoreceptive member such that the developing material is attracted to the charged image areas on the photoconductive surface.
A dry developer mixture usually comprises carrier granules having toner particles adhering triboelectrically thereto. Toner particles are attracted from the carrier granules to the latent image forming a toner powder image thereon. Alternatively, a liquid developer material may be employed. The liquid developer material includes a liquid carrier having toner particles dispersed therein. Generally, the toner is made up of resin and a suitable colorant such as a dye or pigment. Conventional charge director compounds may also be present. The liquid developer material is advanced into contact with the electrostatic latent image and the toner particles are deposited thereon in image configuration.
The developed toner image recorded on the imaging member is then transferred either directly to an image receiving substrate such as paper or first to an intermediate transfer member and then to an image receiving substrate. When a liquid developer material is employed, it is most advantageous to utilize an intermediate transfer member in order to avoid transferring any liquid carrier to an image receiving substrate. In other words, it is advantageous to transfer the developed image to a coated intermediate transfer web, belt or component, and subsequently transfer with very high transfer efficiency the developed image from the intermediate transfer component to a permanent substrate.
The toner particles may be transferred by heat and/or pressure to an intermediate transfer member, or more commonly, the toner particles may be electrostatically transferred to the intermediate transfer member by means of an electrical potential between the imaging member and the intermediate transfer member. After the toner has been transferred to the intermediate transfer member, it is then transferred to the image receiving substrate, for example by contacting the substrate with the toner image on the intermediate transfer member under heat and/or pressure. The intermediate transfer member may also be used as a transfix member, i.e., as also participating in the fusing of the image to the image receiving substrate.
Intermediate transfer members enable high throughput at modest process speeds. In four-color photocopier systems, the intermediate transfer member also improves registration of the final color toner image. In such systems, the four component colors of cyan, yellow, magenta and black may be synchronously developed onto one or more imaging members and transferred in registration onto an intermediate transfer member at a transfer station.
In a final step in the process, the photoconductive surface of the photoreceptive member is cleaned to remove any residual developing material that may be remaining on the surface thereof in preparation for successive imaging cycles.
In electrostatographic printing machines wherein the toner image is electrostatically transferred by a potential between the imaging member and the intermediate transfer member, the transfer of the toner particles to the intermediate transfer member and the retention thereof should be as complete as possible so that the image ultimately transferred to the image receiving substrate will have a high resolution. Substantially 100% toner transfer occurs when most or all of the toner particles comprising the image are transferred and little residual toner remains on the surface from which the image was transferred. Substantially 100% toner transfer is especially important for generating full color images since undesirable shifting or color deterioration in the final colors can occur when the primary color images are not accurately and efficiently transferred to and from the intermediate transfer members.
To increase toner transfer, the resistivity of the intermediate transfer member should be within a desired range, and preferably, wherein the resistivity is virtually unaffected by changes in humidity, temperature, bias field, and operating time. The desired resistivity range of operation varies for different types of electrostatographic printing devices, and thus it is highly desirable to have a transfer member that is comprised of material which is readily tunable in terms of conductivity/resistivity.
Bias members require a resistivity of the entire charging member within a desired range. Specifically, materials with too low resistivities will cause shorting and/or unacceptably high current flow to the photoconductor. Materials with too high resistivities will require unacceptably high voltages. Other problems that can result if the resistivity is not within the required range include low charging potential and non-uniform charging, which can result in poor image quality.
Therefore, it is important in biasable members that the resistivity be tailored to a desired range and that the resistivity remain within this desired range. Accordingly, it is desirable that the resistivity be unaffected or virtually unaffected to changes in temperature, relative humidity, running time, etc.
Attempts at controlling the resistivity of intermediate transfer members have been accomplished by, for example, adding conductive fillers such as ionic additives and/or carbon black to the conformable layer.
U.S. Pat. Nos. 3,959,573 and 3,959,574 describe adding additives such as a quaternary ammonium compound to hydrophobic and hydrophilic elastomeric polyurethane layers, respectively, in order to control the changes in resistivity due to changes in relative humidity. Similarly, U.S. Pat. Nos. 5,286,570, 5,259,990, 5,286,566 and 5,259,989 all describe the addition of an asymmetric ionic quaternary ammonium salt to a polyurethane elastomer to extend the useful electrical life of the polyurethane elastomers.
U.S. Pat. No. 5,112,708 discloses a charging member comprising a surface layer formed of N-alkoxymethylated nylon which may be filled with fluorinated carbon. U.S. Pat. No. 5,000,875 discloses tetrafluoroethylene copolymer compositions containing conductive carbon black or graphite fibers to increase conductivity when the tetrafluoroethylene copolymer has been treated with a fluorinating agent.
U.S. Pat. No. 5,397,863 discloses film capacitors using polyimide materials and fluorinated carbons. U.S. Pat. No. 5,556,899 discloses adding fluorinated carbon to polyimide materials to effect a change in the dielectric constant and the coefficient of thermal expansion of the polyimide for use in electronic packaging. U.S. Pat. No. 5,571,852 discloses use of fluorinated carbon in polyimide materials for electrical conductor patterns. U.S. Pat. No. 5,591,285 discloses adding fluorinated carbon to fluoropolymers and exposing the material to ultraviolet radiation for electronic packaging applications.
U.S. Pat. No. 6,066,400 describes a biasable member having a fluorinated carbon filled polyimide layer which exhibits controlled electrical conductivity, along with embodiments wherein the fluorinated carbon filled polyimide layer is a substrate, embodiments wherein the fluorinated carbon filled polyimide is a substrate having thereon a filled fluoropolymer outer layer, and embodiments wherein the fluorinated carbon filled polyimide layer is a substrate having thereon an intermediate metal layer, and an outer polymer layer.
In the manufacturing process of making such filled polyimide layers, a dispersion of the filler and the prepolymer solution is coated or extruded to form a coated film. The film is then dried, followed by a curing step to imidize the prepolymer to form the polyimide polymer. The process must be conducted at lower temperatures, for example 320xc2x0 C. or below in order to avoid decomposition of the fluorinated carbon materials, which tend to have fairly low thermal stabilities. For example, ACCUFLUOR 2028, a material formerly manufactured by AlliedSignal, starts decomposing at around 320xc2x0 C. One of the decomposition products of these fluorinated carbon materials may be F2. Since F2 is corrosive, manufacturing relaxable substrates using ACCUFLUOR 2028 and like materials is considered unsafe at higher temperatures. As a result of the need to use lower temperatures to imidize or cure the film layer, the imidization may not be as complete as at higher temperatures, and this may result in property differences in the layer, particularly with respect to the mechanical properties.
A need thus remains for an improved process of making fluorinated carbon filled films/layers that may be used as, for example, substrates of an intermediate transfer of transfix member.
It is thus one object of the present invention to develop a novel method of making a polyimide layer containing fluorinated carbon materials that permits imidization of the layer with reduced risk of release of F2.
It is a further object of the present invention to develop an improved polyimide layer containing fluorinated carbon materials that has a readily tunable resistivity so as to be suitable for use as a transfer or transfix member substrate.
These and other objects are achieved by the present invention.
In one embodiment, the present invention relates to a method of making a polyimide layer containing fluorinated carbon materials therein, comprising subjecting a layer comprised of a polyimide precursor material and the fluorinated carbon materials to heating at a temperature less than about 400xc2x0 C. for at least about 1 minute, and thereafter subjecting the layer to further heating at a temperature greater than about 400xc2x0 C. for at least about 1 minute, whereby the polyimide precursor is imidized to a polyimide.
In a further embodiment, the invention relates to a method of making a transfer member or transfix member, comprising forming a substrate by subjecting a layer comprised of a polyimide precursor material and fluorinated carbon materials to heating at a temperature less than about 400xc2x0 C. for at least about 1 minute and thereafter subjecting the layer to further heating at a temperature greater than about 400xc2x0 C. for at least about 1 minute, whereby the polyimide precursor is imidized to a polyimide to form the substrate, and applying a surface coating to the substrate. In a further embodiment, the invention relates to a polyimide layer containing fluorinated carbon materials therein, most preferably made by the above-described method, wherein the fluorinated carbon materials have a thermal decomposition starting temperature of at least about 400xc2x0 C. In a still further embodiment, the polyimide layer is a substrate of a transfer or transfix member.